An evaluation of the Coulter VCS (volume, conductivity, light scatter) automated differential counter demonstrated satisfactory correlations with manual 800-cell differential counts for neutrophils, lymphocytes, monocytes and eosinophils. For the detection of abnormal cells, 12.2% of samples gave false-negative results on the VCS, and 13.4% gave false-positive results. However, only 0.8% of the false-negatives would be expected to be picked up by a standard 100-cell manual differential count. Carry-over accorded to manufacturer's specifications and throughput was 60 samples/h. Reference normal ranges have been established and the instrument's precision and performance with leucopenic and neonatal blood samples assessed.